Biodiesel is a processed fuel that has similar combustion properties to fossil diesel. Biodiesel is derived from the esterification and transesterification of free fatty acids (FFAs) and triglycerides, respectively, which occur naturally in renewable biological sources such as plants oils and animal fats. Since biodiesel can significantly decrease the emission of CO2, SOx, and unburned hydrocarbons from the exhaust of motor vehicles, it is environmentally beneficial, and therefore, a promising alternative to fossil diesel.
The conventional process used for biodiesel production converts triglycerides to alkylesters by transesterification with alcohols in the presence of homogeneous base catalysts as depicted in Equation 1. Even though homogeneous catalyzed biodiesel processes are relatively fast and show high conversion rates, the separation of the homogeneous catalyst from the reaction products can be problematic. Primarily, the aqueous quenching of the reaction may result in the formation of stable emulsions and the saponification of the homogenous catalysts, making the separation of methyl-esters (biodiesel) and glycerol products difficult and costly; requiring the use of a large amount of excess water. Moreover, the use of homogeneous catalysts (strong acids or bases) can lead to many engineering problems, such as equipment corrosion, as well as some environmental problems, such as the disposal of the waste catalysts and contaminated wash water.

Currently, most of the commercial biodiesel produced in the U.S. comes from the transesterification of food grade vegetable oils with methanol using alkaline (KOH, NaOCH,) as catalysts. With this conventional homogeneous method, free fatty acid (FFA) content in oils must be lower than 0.5% and water content lower than 0.06%. Thus, high cost of food grade vegetable oils leads to high production cost of biodiesel.
In order to minimize problems associated with homogeneous catalytic processes, the development of efficient heterogeneous catalyst systems for use in the formation of biodiesel products would be beneficial. The use of a heterogeneous catalyst system could offer multiple advantages over conventional homogeneous catalyst systems. First, heterogeneous catalysts would be more easily recovered from the reaction mixture and recyclable. Second, heterogeneous catalysts, which are typically non-corrosive, would be more environmentally benign.
Although several types of heterogeneous catalyst systems have been evaluated for use in biodiesel production, each of these catalyst systems exhibits various disadvantages or inefficiencies that need to be overcome. For example, although some alkali metal or alkali earth-containing catalysts show high initial activities, these alkali metal ions are easily dissolved in the reaction media. Furthermore, a spinel mixed oxide of Zn and Al, which is commercially available from the French Institute of Petroleum, requires both a high temperature and a high pressure condition when used as a heterogeneous catalyst for biodiesel production due to a relatively low level of catalytic activity. In order for a heterogeneous catalyst to be effectively used for the formation of biodiesel products, it is desirable that the occurrence of the small surface area associated with the catalyst that is caused by sintering at high temperatures and the dissolution of the catalyst into the reaction media be addressed.
The heterogeneous catalyst system of the present disclosure addresses the issues related to homogeneous catalysts and conventional heterogeneous catalyst systems. The heterogeneous catalyst system of the present disclosure utilizes the influences exerted by basic supports (CeO2 or La2O3) to enhance the catalytic behavior of supported metal oxide catalysts (e.g., oxides of Ce, La, Ca, Nd, Pr, Er, Gd, and Yb). Mixed metal oxides of Ce, La, Ca, Nd, Pr, Er, Gd, and Yb may be supported on basic supports of CeO2 or La2O3 in the preparation of heterogeneous catalyst systems for the transesterification of soybean oil with methanol in both batch and continuous flow reactors.